1873.] Histology of the Land-Planarians of Ceylon. 173 



which Mr. Spencer calls secondary aggregates. If Mr. Spencer's hypo- 

 thesis was correct, we should expect to find at least some Annelid deve- 

 loping its segments in the egg as a series of buds. It is not, of course, 

 here meant to be concluded that Annelids are not sometimes in a 

 condition of tertiary aggregation, as Nais certainly is when in a budding 

 condition, but that ordinarily they are secondary and not tertiary aggre- 

 gates ; and if so, then so also are Arthropoda. 



Much more information concerning the anatomy of Planarians will be 

 required before it will be possible to trace the line of descent of Bipalium 

 and Bhynchoclemus, and determine what was the form of their aquatic 

 ancestors. In the absence of accurate accounts of the structure of the 

 American Land-Planarians, and even of the European Bhynchoclemus 

 terrestris, the question is very puzzling. The formation of either one of 

 the two forms Bipalium or Bhynchoclemus might be accounted for with 

 comparative ease, from the arrangement of parts in the flat head of 

 Bipalium. Prom the tree-like branching of the digestive tract in that 

 region, the corresponding ramification of the vascular system, and general 

 muscular arrangement, it might be imagined that Bipalium had come 

 from a flattened parent of the common Planarian form, and that all the 

 body except the head had become rounded and endowed with an 

 ambulacral line. In nearly all points, except the eyes and the absence 

 of branches to the oviduct, Bipalium seems more highly specialized than 

 Bhynchoclemus. We might imagine that Bhynchoclemus and Bipalium had a 

 common parent, and that when an ambulacral line was just beginning to 

 be developed, the two forms took different lines — Bhynchoclemus losing all 

 traces of the original flatness of its ancestor, and never developing any 

 ciliated sacs or papillae, but cherishing a single pair of large eyes at the 

 expense of all the rest which it possessed, its testes, moreover, remaining 

 in a comparatively primitive condition. But then comes the difficulty 

 about the great difference in shape in the pharynxes of the two forms ; 

 and if it be suggested that, as is highly probable, several or many aquatic 

 Planarians have taken to terrestrial habits, and that Bipalium has been 

 derived from a form like Leptoplana, with a folded pharynx, whilst 

 Bhynchoclemus came from an ancestor with a tubular one, it is difficult 

 to account for the many points of close resemblance between these two 

 forms, and especially their similarity in external colouring, though this 

 latter may perhaps be explained by mimicry. On the whole, it is evident 

 that a close study of the anatomy of Land-Planarians cannot fail to lead 

 to interesting results, and it is hoped that this memoir may lead to 

 further work of the same kind. It would be of especial value to have a 

 good account of the anatomy of Geoclesmus and Bhynchoclemus sylvaticus. 



